iPS cells possess capabilities of differentiation and tissue formation equivalent to those of embryonic stem cells (ES cells). Because of inducibility from human primary culture cells, iPS cells are cells potentially possessing the capability of playing a central role in regenerative medicine. Yamanaka et al. established an iPS cell that possesses pluripotency as do ES cells by transferring four factors (Oct3/4, Sox2, Klf4, c-Myc) to mouse embryonic fibroblasts (MEF) (Non-patent Document 1). In addition to MEF, they succeeded in establishing mouse iPS cells from other various cells (Non-patent Documents 2 and 3), organs (Non-patent Document 4) and the like. Furthermore, in humans as well, establishment of iPS cells from human somatic cells using the same technique was reported (Non-patent Documents 5-8).
An aspect that can be a major barrier to ensuring the efficacy and safety of iPS cells in the context of their clinical application resides in the diversity thereof. It is speculated that the functioning of iPS cells differs among different lines depending on the individual's genetic background, the type of cells, the degree of reprogramming, the stage of genesis at which the cells are immortalized, and the like. In fact, even in mouse ES cells, the gene expression pattern differs widely depending on the genetic background, and the differentiation competence varies widely among different lines. It has already been found by Yamanaka et al. that in human iPS cells as well, the gene expression pattern differs widely among different lines (Non-patent Document 5). Therefore, it is anticipated that the differentiation competence and tumorigenesis tendency vary considerably among different iPS cells.
Also, there is a room for further improvement in the nuclear reprogramming protocol; various improved protocols have been reported (Non-patent Documents 9-14).
The hematopoietic-immune system has long been positioned as a subject of regenerative medicine or cytotherapy, from blood transfusion to bone marrow transplantation and cord blood transplantation, and these therapies have been shown to be substantially effective. It has also been shown that in mice and humans, a variety of hematopoietic-immune system cells can be differentiation-induced from ES cells. This shows that induction of hematopoietic-immune system cells from iPS cells would be potentially effective as a therapy within the conventional framework.
Natural killer T (NKT) cells, which is in a series of lymphocytes that constitute the immune system, play a central role in antitumor immunity and defensive reactions to infectious diseases by the adjuvant effect of the Th1 cytokine IFN-γ; the lack of NKT cells leads to infectious death. Meanwhile, because immune responses are controlled by IL-10, which is a Th2 cytokine produced by NKT cells, the cells are also responsible for control of transplantation immune reaction and suppression of the onset of autoimmune diseases. As stated above, NKT cells occupy the central part in immune control; therefore, α-galactosyl ceramide (α-GalCer), which is a glycolipid ligand of NKT cells that allows artificial control of the cellular function thereof, is not the only currently available immune response regulator; in fact, NKT cells are already in some clinical applications in lung cancer patients, and the results of phase II clinical studies that have been conducted to date show that the mean survival time is extended as much as 4-5 times compared with chemotherapy.
However, in cancer patients who are subjects of NKT cell immunotherapy, NKT cell count reductions and functional defects are seen (see, for example, Non-patent Document 15), so that there are some cases in which activation of NKT cells sufficient to obtain a therapeutic effect cannot be accomplished merely by administration of α-GalCer-pulsed dendritic cells. In this case, it is possible that NKT cells are collected from the patient or another person of the same HLA type and expanded, and then returned (transplanted) to the patient; however, NKT cells are usually present in extremely small amounts at not more than 0.1% of peripheral blood lymphocytes, and it is uneasy to mass-expand them. Therefore, provided that an NKT cell clone can be expanded in large amounts by reprogramming the NKT cells collected, and then allowing them to differentiate and mature again, it is expected that the therapeutic effect of NKT cell immunotherapy can be improved.
However, finally differentiated cells are less easy to reprogram, compared with undifferentiated cells; no reports of induction of iPS cells from T cells or NKT cells have been presented to date. For B cells, it was reported that iPS cells cannot be induced with what are called the four factors or three factors (Oct3/4, Sox2, Klf4) only, their induction often requiring the use of another gene as a nuclear reprogramming factor (Non-patent Document 3). However, increasing the number of transgenes is feared to intensity safety concerns, including the potential tumorigenesis of the cells differentiated from iPS cells. Furthermore, even if iPS cell were established from an NKT cell, it would remain unknown whether differentiation from the iPS cell to functional mature NKT cells could be induced.